Gas tube overvoltage protectors are widely used for the protection of equipment from overvoltage conditions which may be caused by lightning, high voltage line contact, and the like.
It is also a widely practiced technique to associate various fail-safe arrangements with such tubes and with other types of protectors, e.g., air gap arresters, to meet various contingencies. For example, the presence of a sustained overload, as where a power line has come in continued contact with a protected telephone line, produces a concomitant sustained ionization of the gas tube and the resultant passage of heavy currents through the tube. Such currents will in many cases destroy the overvoltage protector and may also constitute a fire hazard.
One common approach to this problem is to employ fusible elements which fuse in the presence of such overloads and provide either a permanent short circuiting of the arrester directly, or function to release another mechanism, e.g. a spring loaded shorting bar, which provides the short circuit connection (commonly, the arrester electrodes are both shorted and grounded). The presence of the permanent short and ground condition serves to flag attention to that condition thus signalling the need for its inspection or replacement. Examples of this type of fail-safe protection are found in U.S. Pat. Nos. 3,254,179; 3,281,625; 3,340,431; 3,396,343; and 3,522,570. Several of these patents also incorporate with the fail-safe feature, a backup air gap arrangement so that there is both fail-safe fusible (short) type protection as well as backup air gap protection.
Still another approach, disclosed in commonly assigned application Ser. No. 719,077 filed Aug. 31, 1976, is based on the discoveries that an effective fail-safe function can be achieved by employing a non-metallic fusible material and that important advantages are consequently realized. The fusible material is an electrical insulator which in the exemplary emobodiments is interposed between one or more of the electrodes and the shorting mechanism. Surprisingly, the response of the nonmetallic material to thermal conditions is precise and, moreover, does not leave an insulative film in the course of fusing which might otherwise interfere with the short circuit contact.
The need exists, nonetheless, to develop fail-safe arrangements which provide both surge and failure protection for gas tube arresters.